Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner onto an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal ink jet (TIJ)) and piezo electric transducers. In another process known as continuous ink jet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions used in ink jet printers can be classified as either pigment-based in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigment-based inks are often preferred over dye-based inks because they possess better fade resistance to light and gas, especially ozone, as compared to printed images with dye-based inks. The pigment particles of pigment-based ink compositions, when printed onto photo-glossy receivers suitable for photographic image quality, typically reside at the surface of the receiver material. Images produced from such pigmented inks exhibit high optical density and good gloss when the dispersion particle sizes are less than about 130 nanometers. When the dispersion pigment particle sizes are less than about 75 nanometers extremely high gloss can be achieved. Image durability is very good when the pigmented inks are comprised of polymeric dispersions or contain additional binders. However, the same pigmented ink compositions, when printed onto uncoated (plain) papers typically used in the home or office environment for routine printing, often provide less density and inferior density consistency as compared to dye-based inks. This is a result of the pigment particles of the pigment-based ink compositions migrating into the interior of the plain paper. The extent of this particle migration varies from paper to paper and depends on the plain paper manufacturing process. Consequently, the pigment particles printed onto such papers receive reduced illumination by incident light, and light scatter that occurs within the plain paper further diminishes the density formed by the pigment particles. In contrast, appreciable colorant in the dye-based ink compositions is absorbed by the paper fibers at or near the surface of the plain paper, which results in higher optical density.
The process of preparing pigment-based ink compositions usually involves two sequential steps: (a) a milling step, conducted in the presence of a dispersing agent, to break up crude pigment aggregates into primary pigment particles that are stabilized by the dispersing agent, and (b) an ink formulation step in which the stabilized pigment dispersion particles are diluted with ink components such as water and water miscible organic compounds (humectants, surfactants, binders, etc.). It is well known in the art that the choice of dispersant in the milling step is critical as it facilitates de-aggregation of the pigment agglomerates and stabilization of the pigment particles as they are being broken up by the mechanical and kinetic energy being provided in the milling process. The choice of dispersant ultimately affects the primary particle size achievable and also determines many of the final physical properties of the dispersion, such as viscosity and surface tension. The dispersant also strongly influences the stability of the dispersion to various ink additives, the jetting quality of the ink, and the final printed image resistance to degradations associated with wet and dry physical abrasion, light and gas fade, and water-fastness.
In the case of organic pigments an efficient dispersion process usually provides a dispersion that exhibits the primary particle sizes associated with the pigment (primary pigment particle sizes are usually determined by the chemical nature of the pigment and its preparative process). Industrial carbon carbon black pigments, however, have many important physical and chemical parameters that can affect the dispersion particle sizes, recorded image quality, and ink performance on various receivers. Important parameters, as described in Carbon Black, Science and Technology, 2d, ed., J. Donnet et al, Marcel Dekker, Inc. 1993, include, primary particle size, aggregate structure as determined by DBP (dibutyl phthalate) oil absorption, and volatiles content. These parameters are determined by the carbon black manufacturing process and any oxidative post treatments the carbon may be subjected to such as those described in U.S. Pat. Nos. 3,347,632; 5,609,671; 5,718,746; 5,928,419; and 6,503,311. For instance, it is known that channel and gas blacks have high DBP oil absorption and high volatiles content whereas furnace blacks have low DBP oil absorption and low volatiles content. Typical DPB absorption values for pigment blacks utilized in printing inks range from about 45 to 125. Carbon pigments, especially, have found wide application in the printing of black text, graphics and photographic images. Today, virtually all black pigment-based ink compositions used in the printing of text, graphics or most photographic quality images are comprised of carbon pigment dispersions of particle size in the sub-micron range.
For the printing of high quality black and white photographic images, writing systems utilizing pigment-based inks sometimes rely on the use of so-called process blacks (printing of CMY pigments) to form a neutral color or, alternatively, on the use of black ink comprised of a surfactant- or polymer-dispersed carbon black for the imaging of neutral colors. Conversely, beginning in the early 1990's much of the desktop ink jet printing industry has adopted the use of black inks comprised of surface functionalized, self-dispersed carbon black pigments for the printing of black text as described in U.S. Pat. No. 5,630,868 to J. Belmont et al, U.S. Pat. No. 5,672,198 to J. Belmont and, in T. Lüthge, G. Tauber, R. McIntosh, W. Kalbitz, and S. Lüdtke, Proc. NIP 20: Int. Conf. on Digital Printing Tech., 2004, IS&T, Springfield, Va., pp 753-757. Pigment inks comprised of self-dispersed black dispersions have provided high text density owing to the efficient aggregation and retention of the carbon black pigment particles at the surface of almost all types of image receivers (plain papers or photo-glossy). As described in U.S. Pat. No. 5,630,868 or Lüthge et al, black inks comprised of such carbon black dispersions exhibit higher optical densities on plain papers as compared to inks comprised of surfactant or polymer dispersed carbon black pigments.
In the case of printing of high quality photographic images onto photo-glossy receivers, high gloss and good durability (resistance to image smear) are of paramount importance whereas the requirements associated with printing of black text/graphics onto plain or other uncoated papers usually center on high visual density and good water fastness and smear resistance. Unfortunately, inks comprised of self-dispersed carbon black pigments such as those disclosed in U.S. Pat. No. 5,630,868, and Lüthge et al, that are highly suitable for text printing show poor smear resistance on uncoated papers and these same inks lack sufficient gloss and image durability to be useful for the printing of high quality photographic images onto photo-glossy papers. Pigment inks that are suitable for printing of photographic quality images onto photo-glossy papers lack the density and density uniformity across the various plain papers often used in desktop ink jet printing. As a consequence ink jet writing systems typically possess a black ink dedicated to the printing of black text/graphics and a separate set of inks dedicated to photo-printing
Thus, a major challenge for pigmented ink compositions comprised of polymer-dispersed pigment particles is to provide high density when printed onto uncoated (plain) papers, while simultaneously providing high density, gloss, and image durability on glossy papers and photo-glossy ink jet receivers. One such approach is to use a mixture of pigments dispersed by dissimilar means in the same ink as described in U.S. Patent Application Number 2005/0223938A1. However, this approach has the drawback that two separate pigment dispersions must be manufactured and formulated into a single ink composition.
In addition to the art cited above additional approaches that utilize high molecular weight polymeric dispersants and latexes in the pigmented ink compositions have been employed in drop-on-demand piezoelectric printers directed at photographic quality pictorial image reproduction (U.S. Pat. No. 6,713,531 B2; U.S. Pat. No. 6,180,691 B1; U.S. Pat. No. 6,866,707 B2). Such inks provide high density on uncoated papers and high density, durable images of modest gloss on photo-glossy receivers. Unfortunately, pigmented inks of such compositions cannot be utilized in thermal ink jet since heaters become easily fouled and small diameter nozzles clogged. Other approaches relying on polymeric dispersants containing high hydrophobic to hydrophilic monomer ratios, not suited for aqueous milling processes, have also been disclosed (U.S. Pat. No. 5,798,426 A issued to Anton et al.). Approaches to modify the uncoated paper compositions by incorporating cationic species capable of binding the pigment particles near the surface of the uncoated receiver have also been employed. Such approaches typically increase the cost of the receiver or complicate the paper making process and have not generally been utilized in uncoated receivers commonly used in the home or office.
U.S. Patent Application 2005/0143491 describes certain block copolymers to be utilized in a non-aqueous ink jet ink. U.S. Pat. No. 5,679,138 A, U.S. Pat. No. 5,651,813 A, and U.S. Pat. No. 5,985,017 A describe the preparation of aqueous pigment-based ink compositions for ink jet printing wherein pigment particles are dispersed with surfactants and have an average particle diameter of less than 100 nm. Although these ink compositions are very useful for photographic-quality imaging, the ink compositions therein may not possess sufficient durability on photo-glossy receivers nor provide high density on a variety of receivers, including uncoated papers. U.S. Patent 2005/0004261 A1, U.S. Patent 2004/0127619 A1, U.S. Pat. No. 6,245,832 B1, U.S. Pat. No. 5,085,698 A, and U.S. Pat. No. 4,597,794 A describe aqueous pigment-based ink compositions for ink jet printing wherein pigment particles are dispersed with polymeric materials derived from hydrophobic and hydrophilic monomers. Use of these dispersants and milling processes provides dispersions that frequently have particle sizes equal to or greater than 100 nm. Pigment-based ink compositions produced from such dispersions do not yield the necessary density on uncoated papers. U.S. Ser. No. 10/891,316, filed Jul. 14, 2004, describes the preparation of aqueous pigmented ink compositions for ink jet printing wherein the pigment particles are dispersed with random copolymers and have an median particle diameter less than about 100 nm. While these pigment-based ink compositions provide high durability and density in drop-on-demand image printing applications, they lack the necessary density on uncoated papers. It is not appreciated in the art that the choice of polymeric dispersant, especially the choice of certain monomer types, will markedly influence the interaction of the pigment particles in pigment-based ink compositions with the surface of uncoated papers.
Generally, polymeric dispersants have been selected for their ability to provide stable pigment dispersions and pigmented ink compositions that enable improved performance on photo-glossy receivers while providing good jetting quality within the printing architecture they must operate. Examples of carbon blacks dispersed with polymers are described in U.S. Pat. Nos. 5,184,148; 5,686,508; 5,919,294; 6,383,278; 6,551,394 and U.S. Patent Application Number 2005/0042164A1. Typically, inks comprised of such polymer-dispersed carbon black pigments lack sufficient density and density uniformity when printed onto uncoated papers. In addition, it is not appreciated in the art that key physical and chemical parameters related to carbon black when polymer dispersed with specific polymer of structures described in U.S. Ser. No. 11/206,588, filed Aug. 18, 2005, can greatly enhance the interaction with and aggregation of carbon black particles on the surface of uncoated papers.
There remains the need for 1) a single black ink, useful in thermal and piezoelectric ink jet drop-on-demand printing and continuous ink jet printing, in which the black ink is able to meet all of the important requirements for plain paper printing while at the same time satisfying the important requirements for photo-printing on glossy receivers; 2) a black ink for text and graphics printing which is water-fast and smear resistant; 3) a black ink of carbon black dispersion with an average particle size of less than about 125 nm for improved jetting quality and reliability in ever-decreasing print head nozzle diameters; and 4) a “universal” black pigment-based ink that can simplify system design and enable opportunities for new hardware and writing system architectures.